1. Field of the Invention
The present invention relates generally to video signal processing systems for processing a reproduced video signal suitable for use with a reproducing system of a professional VTR (video tape recorder). More particularly, this invention relates to a video signal processng system for processing a reproduced video signal suitable for use in a reproducing mode with a variable tape speed very close to a normal tape speed.
2. Description of the Prior Art
When a video signal is reproduced by, for example, a VTR at a variable tape speed very close to the normal tape speed, for example when a video signal one hour and one minute long is reproduced during the period of one hour, a non-coincidence between an odd or even field of the reproduced video signal and a reference odd or even field appears sometimes. If such reproduced video signal is reproduced as is by a monitor television receiver, a flicker occurs in the vertical direction of the reproduced picture and the reproduced picture becomes difficult to see.
Therefore, in the prior art, when the odd and even fields of the reproduced video signal are coincident with the reference off and even fields, they are left as they are, while when they are not coincident, adjacent line signals of the reproduced video signal are averaged arithmetically and then used.
This signal processing will be described with reference to FIGS. 1 and 2. FIG. 1 illustrates a signal of a certain field of the reproduced video signal. In FIG. 1, reference numerals P1 to P4 designate levels of some points on sequential lines .lambda., .beta., .gamma. and .delta., wherein the levels P1 and P2 are represented as normalized level 0 while the levels P3 and P4 as normalized level 1, and the level of the video signal is changed from the normalized level 0 to 1 at a point intermediate the adjacent lines .beta. and .gamma.. In this case, x is a coordinate indicative of the location of the line.
When a signal of another field is produced from the signal of the certain field of the reproduced video signal by the above mentioned arithmetic mean processing, it becomes as shown in FIG. 2. In FIG. 2, reference numerals Q1 to Q3 indicate levels of points on sequential lines A, B and C (corresponding to the intermediates of the lines .alpha., .beta., .gamma. and .delta. of FIG. 1). The level Q1 becomes the normalized level 0, the level Q2 the normalized level 0.5 and the level Q3 the normalized level 1, respectively. In this case, z is a coordinate indicating the location of the line.
In this case, a level Y (z) of a point on a certain line z is expressed by the following equation. EQU Y(z)=Y(z+1/2)/2+Y(z-1/2)/2
If the video signal is processed in this way as in the prior art or a signal of another field is produced from a signal of a certain field of the video signal by arithmetic mean processing, the signal of the resultant field is lowered in the sharpness below that of the signal of the original fields. As a result, the reproduced picture is changed in sharpness and hence the reproduced picture becomes difficult to see.
Therefore, in order to reduce the change of sharpness, it may be considered that the inclination of the level of the original field of the video signal is matched with that of the level of the field of the averaged signal. Levels of the points on the respective lines .alpha., .beta., .gamma. and .delta. of the signal of the field with level inclination changed become as shown by P1, P5, P6 and P4 in FIG. 1. In this case, level Y (z) of a point on a certain line z is expressed by the following equation. EQU Y(x)=Y(x)/2+Y(x x+1)/4+Y(x-1)/4
Accordingly, for an inclination with the normalized level of about 0.5, the change of the video signal from the levels P5 to P6 of FIG. 1 becomes the same as the change of the video signal from the levels Q1 - Q2 - Q3 of FIG. 2.
However, in practice, the user does not notice the change of the video signal from the levels P5 to P6 of FIG. 1 but rather the change close to the change of the video signal from the levels P1 - P5 - P6 - P4 of FIG. 1. This means that the sharpness of the reproduced picture is lowered.